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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

76
ALASKAN GLACIER STUDIES
July, 1909. The surface of the Baird Glacier of southeastern Alaska was lowered an inch a day by ablation in 1898. Muir Glacier surface melted 2 inches a day in 1890 and 1892, On the broken Atrevida, much nearer the sea, and at a lower elevation, the rate of ablation must have been greater than that on the Hayden. But when the ice surface began again to be sheeted with moraine the rate of ablation must have notably decreased. With these variable factors, and the absence of exact measurements, we can do no more than make the general statement that ablation, at first rapid, then at a decreasing rate, had so-greatly lowered the surface of the Atrevida Glacier between August, 1906 and July, 1909^ as to partially heal the crevasses that resulted from the spasmodic advance of 1906 so that it was possible to once more travel over its surface.
Specific measurements of the thickening of Atrevida Glacier through the 1906 advance, and the subsequent thinning through ablation before 1909 are based upon a pair of cross-
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FIG. 5.   CHOBS-SBCTIONS OP ATBHVTDA GLAOTBB.
sections of Atrevida Glacier which the junior author made, one in 1905 before the advance had commenced, the other in 1909. The 1905 and 1909 elevations are barometric. These are shown in the three profiles of Fig. 5. The slopes and elevations of the 1906 cross-section are hypothetical. Each cross-section is from Esker Stream at the base of Amphitheatre Knob on the east side of Atrevida Glacier to Terrace Point on the west side. It will be seen that the 1909 cliff at Esker Stream is 125 feet higher than that of 1905. Assuming ablation here of 50 to 100 feet we should have had an ice cliff in 1906 of 400 to 450 feet. The thickening would therefore be 175 to 225 feet at least. In the middle of the glacier the thickening, similarly computed, must have been at least 130 to 180 feet. These comparisons, though in part based upon rough estimates, clearly indicate great thickening, and may safely be considered minimum measurements, since the amount of ablation is unknown, our calculation being based upon average depth of crevasses, to or near the bottoms of which ablation has now reduced the glacier surface. The margin of Atrevida Glacier was examined both at and near Esker Stream and at